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[@@deriving yojson]

ppx_yojson_conv is a PPX syntax extension that generates code for converting OCaml types to and from Yojson.Safe, as defined in the =yojson= library. Yojson.Safe are defined by the following type:

type t =
    [ `Null
    | `Bool of bool
    | `Int of int
    | `Intlit of string
    | `Float of float
    | `String of string
    | `Assoc of (string * t) list
    | `List of t list
    | `Tuple of t list
    | `Variant of string * t option

and are rendered as normal json format.

ppx_yojson_conv fits into the =ppx_deriving= framework, so you can invoke it the same way you invoke any other deriving plug-in. Thus, we can write

type int_pair = (int * int) [@@deriving yojson]

to get two values defined automatically, yojson_of_int_pair and int_pair_of_yojson. If we only want one direction, we can write one of the following.

type int_pair = (int * int) [@@deriving yojson_of]
type int_pair = (int * int) [@@deriving of_yojson]

Note that Yojson-converters for primitive types like int need to be brought into scope in order for the ppx to use them, for example by adding open Ppx_yojson_conv_lib.Yojson_conv.Primitives at the beginning of the file.

It’s also possible to construct converters based on type expressions, i.e.:

[%yojson_of: (int * string) list] [1,"one"; 2,"two"]
|> Yojson.Safe.to_string;;
=> {|[[1,"one"],[2,"two"]]|}

[%yojson_of: (int * string) list] [1,"one"; 2,"two"]
|> [%of_yojson: (int * string) list];;
=> [1,"one"; 2,"two"]

For %yojson_of, we can also omit the conversion of some types by putting underscores for that type name.

[%yojson_of: (int * _) list] [1,"one"; 2,"two"]
|> Yojson.Safe.to_string;;
=> {|[[1,"_"],[2,"_"]]|}

Conversion rules

In the following, we’ll review the serialization rules for different OCaml types.

Basic types

For numbers like int, int32, int64, the value is stored as `Int value. For float number, the value is stored as `Float value. For the types char or string, the value is stored as `String str where str is respectively a one character string or the string itself.

Lists and arrays

OCaml-lists and arrays are represented as Yojson.Safe lists.

Tuples and unit

OCaml tuples are treated as lists of values in the same order as in the tuple. The type unit is treated as Yojson `Null. e.g.:

(3.14, "foo", "bar bla", 27)  =>  [3.14, "foo", "bar bla", 27]


With options, None is treated as Yojson `Null, and Some is treated as the value contained, as shown below.

None        =>  `Null
Some value  =>  value

The rules for variants are described below.


Records are represented as Yojson `Assoc (string * t) list, where item of the list is a key-value pair. Each pair consists of the name of the record field (first element), and its value (second element). e.g.:

{ foo = (3,4);
  bar = "some string"; }
=>  {"foo":[3,4],"bar":"some string"}

Type specifications of records allow the use of several attributes. The attribute yojson.option indicates that a record field should be optional. e.g.:

type t =
  { x : int option;
    y : int option [@yojson.option];
  } [@@deriving yojson]

The following examples show how this works.

{ x = Some 1; y = Some 2; } => {"x":1,"y":2}
{ x = None  ; y = None;   } => {"x":null}

When the JSON object keys differ from the ocaml field names, users can specify the corresponding JSON key implicitly using [@key "field"], for example:

type t = {
  typ    : float [@key "type"];
  class_ : float [@key "CLASS"];
[@@deriving yojson, yojson_fields]

The yojson_fields attribute generates the list of JSON keys from a record type, for example:

type ty = {
  x : float [@key "a"];
  y : float [@key "b"];
  z : float
[@@deriving yojson_fields]

generates the list below, and the list will not be generated for the signature.

yojson_fields_of_ty = ["a"; "b"; "z"]

Note that ppx_deriving_yojson support duplicated fields, while our library does not.


More complex default values can be specified explicitly using several constructs, e.g.:

type t =
  { a : int [@default 42];
    b : int [@default 3] [@yojson_drop_default (=)];
    c : int [@default 3] [@yojson_drop_if fun x -> x = 3];
    d : int list
  } [@@deriving yojson]

The @default annotation lets one specify a default value to be selected if the field is not specified, when converting from Yojson.Safe. The @yojson_drop_default annotation implies that the field will be dropped when generating the Yojson.Safe if the value being serialized is equal to the default according to the specified equality function. @yojson_drop_if is like @yojson_drop_default, except that it lets you specify the condition under which the field is dropped.

Specifying equality for [@yojson_drop_default]

The equality used by [@yojson_drop_default] is customizable. There are several ways to specify the equality function:

type t =
  { a : u [@default u0] [@yojson_drop_default (=)]; (* explicit user-provided function *)
    b : u [@default u0] []; (* uses [%compare.equal: u] *)
    c : u [@default u0] [@yojson_drop_default.equal]; (* uses [%equal: u] *)
    d : u [@default u0] [@yojson_drop_default.yojson]; (* compares yojson representations *)
    e : u [@default u0] [@yojson_drop_default]; (* deprecated. uses polymorphic equality. *)
  } [@@deriving yojson]

Allowing extra fields

The @yojson.allow_extra_fields annotation lets one specify that the yojson-converters should silently ignore extra fields, instead of raising. This applies only to the record to which the annotation is attached, and not to deeper yojson converters that may be called during conversion of a yojson to the record.

type t = { a: int } [@@deriving yojson]
{"a":1,"b":2} => exception

type t = { a: int } [@@deriving yojson] [@@yojson.allow_extra_fields]
{"a":1,"b":2} => {a = 1}

type t = A of { a : int } [@yojson.allow_extra_fields] [@@deriving yojson]
["A", {"a":1,"b":2}] => A {a = 0}


Constant constructors in variants are represented as a list with one string, which is the name of the contructor. Constructors with arguments are represented as lists, the first element being the constructor name, the rest being its arguments. For example:

type t = A | B of int * float * t [@@deriving yojson]
B (42, 3.14, B (-1, 2.72, A))  =>  ["B",42,3.14,["B",-1,2.72,["A"]]]

The above example also demonstrates recursion in data structures.

if the JSON variant names differ from OCaml conventions, users can specify the corresponding JSON string explicitly using [@name "constr"], for example:

type t =
| Typ   [@name "type"]
| Class [@name "class"]
[@@deriving yojson]

Polymorphic variants

Polymorphic variants behave almost the same as ordinary variants. The notable difference is that polymorphic variant constructors must always start with an either lower- or uppercase character, matching the way it was specified in the type definition. This is because OCaml distinguishes between upper and lowercase variant constructors. Note that type specifications containing unions of variant types are also supported by the Yojson converter, for example as in:

type ab = [ `A | `B ] [@@deriving yojson]
type cd = [ `C | `D ] [@@deriving yojson]
type abcd = [ ab | cd ] [@@deriving yojson]

However, because `ppx_yojson_conv` needs to generate additional code to support inclusions of polymorphic variants, `ppx_yojson_conv` needs to know when processing a type definition whether it might be included in a polymorphic variant. `ppx_yojson_conv` will only generate the extra code automatically in the common case where the type definition is syntactically a polymorphic variant like in the example above. Otherwise, you will need to indicate it by using `[@@deriving yojson_poly]` (resp `of_yosjon_poly`) instead of `[@@deriving yojson]` (resp `of_yojson`):

type ab = [ `A | `B ] [@@deriving yojson]
type alias_of_ab = ab [@@deriving yojson_poly]
type abcd = [ ab | `C | `D ] [@@deriving yojson]

Polymorphic values

There is nothing special about polymorphic values as long as there are conversion functions for the type parameters. e.g.:

type 'a t = A | B of 'a [@@deriving yojson]
type foo = int t [@@deriving yojson]

In the above case the conversion functions will behave as if foo had been defined as a monomorphic version of t with =’a= replaced by int on the right hand side.

If a data structure is indeed polymorphic and you want to convert it, you will have to supply the conversion functions for the type parameters at runtime. If you wanted to convert a value of type =’a t= as in the above example, you would have to write something like this:

yojson_of_t yojson_of_a v

where yojson_of_a, which may also be named differently in this particular case, is a function that converts values of type =’a= to a Yojson. Types with more than one parameter require passing conversion functions for those parameters in the order of their appearance on the left hand side of the type definition.

Opaque values

Opaque values are ones for which we do not want to perform conversions. This may be, because we do not have Yojson converters for them, or because we do not want to apply them in a particular type context. e.g. to hide large, unimportant parts of configurations. To prevent the preprocessor from generating calls to converters, simply apply the attribute yojson.opaque to the type, e.g.:

type foo = int * (stuff [@yojson.opaque]) [@@deriving yojson]

Thus, there is no need to specify converters for type stuff, and if there are any, they will not be used in this particular context. Needless to say, it is not possible to convert such a Yojson back to the original value. Here is an example conversion:

(42, some_stuff)  =>  [42,"<opaque>"]


Unlike Sexp deriver, we are not handling exceptions in the yojson derivier.

Hash tables

The Stdlib’s Hash tables, which are abstract values in OCaml, are represented as association lists, i.e. lists of key-value pairs, e.g.:


Reading in the above Yojson as hash table mapping strings to integers ((string, int) Hashtbl.t) will map foo to 42 and bar to 3.

Note that the order of elements in the list may matter, because the OCaml-implementation of hash tables keeps duplicates. Bindings will be inserted into the hash table in the order of appearance. Therefore, the last binding of a key will be the “visible” one, the others are “hidden”. See the OCaml documentation on hash tables for details.

A note about signatures

In signatures, ppx_yojson_conv tries to generate an include of a named interface, instead of a list of value bindings. That is:

type 'a t [@@deriving yojson]

will generate:

include Yojsonable.S1 with type 'a t := 'a t

instead of:

val t_of_yojson : (Yojson.Safe.t -> 'a) -> Yojson.Safe.t -> 'a t
val yojson_of_t : ('a -> Yojson.Safe.t) -> 'a t -> Yojson.Safe.t

There are however a number of limitations:

  • the type has to be named t
  • the type can only have up to 3 parameters
  • there shouldn’t be any constraint on the type parameters

If these aren’t met, then ppx_yojson_conv will simply generate a list of value bindings.